Fix occasional crashes with DNSSEC and large nunbers of --address configs.

Commit 3e659bd4ec removed the concept of
an usptream DNS server which is capable of DNSSEC: they are all
(at least in theory) now usable. As a very unfortunate side-effect,
this removed the filter that ensured that dnssec_server() ONLY
returns servers, and not domains with literal addresses.

If we try and do DNSSEC queries for a domain, and there's
a --address line which matches the domain, then dnssec_server()
will return that. This would break DNSSEC validation, but that's
turns out not to matter, because under these circumstances
dnssec_server() will probably return an out-of-bounds index into
the servers[] array, and the process dies with SIGSEGV.

Many thanks to the hard workers at the Tomato project who
found this bug and provided enough information to diagnose it.

src/domain-match.c

@@ -467,7 +467,7 @@ int dnssec_server(struct server *server, char *keyname, int *firstp, int *lastp)
   /* Find server to send DNSSEC query to. This will normally be the 
      same as for the original query, but may be another if
      servers for domains are involved. */		      
-  if (!lookup_domain(keyname, F_DNSSECOK, &first, &last))
+  if (!lookup_domain(keyname, F_SERVER | F_DNSSECOK, &first, &last))
     return -1;
 
   for (index = first; index != last; index++)